This invention relates to a novel ophthalmic and optical glass of a B.sub.2 O.sub.3 -SiO.sub.2 -Li.sub.2 O-CaO-La.sub.2 O.sub.3 -TiO.sub.2 -ZrO.sub.2 -Nb.sub.2 O.sub.5 system which has wide ranges of optical constants, namely a refractive index (.eta.d) in the range of 1.75 to 1.84, Abbe number (.nu.d) in the range of 33 to 40 and specific gravity of 4.0 or less, and is excellent in resistivity to devitrification and is capable of being imparted with a high impact resisting property in a chemical aftertreatment.
For an ophthalmic lens, a lens of a refractive index (.eta.d) of about 1.52 has often been used. It is disadvantageous, however, to produce a lens of a high positive or negative diopter value by using a lens of such a low refractive index, for the peripheral or central portion of the lens becomes thick and therefore the weight of the lens increases and, in addition, chromatic aberration in the peripheral portion increases thereby causing occurrence of a rainbow with resulting disadvantages for the user of the ophthalmic lens both in use and appearance.
For eliminating such disadvantages, proposals have been made for providing a glass of a high refractivity and low specific gravity and various ophthalmic glasses having a high refractive index .eta.d.gtoreq. about 1.75 have become known. For example, U.S. Pat. No. 4,213,786 discloses a glass of SiO.sub.2 -B.sub.2 O.sub.3 -CaO-Sro-TiO.sub.2 -La.sub.2 O.sub.3 system. This glass, however, has the disadvantage that it tends to become tinted. The U.S. Pat. No. 4,612,295 discloses a SiO.sub.2 /B.sub.2 O.sub.3 -CaO-Nb.sub.2 O.sub.5 system glass in which the SiO.sub.2 ingredient should be comprised as a main ingredient in the principal ingredients of SiO.sub.2 and B.sub.2 O.sub.3. This glass, however, is not suitable for mass production because it has only poor resistivity to devitrification. Besides, it is difficult to impart an impact resisting property to ophthalmic glasses produced from these glasses by a chemical strengthening aftertreatment and hence safety in using these ophthalmic glasses is not ensured. U.S. Pat. No. 4,213,787 discloses a SiO.sub.2 -Li.sub.2 O-BaO-TiO.sub.2 -ZrO.sub.2 -Nb.sub.2 O.sub.5 system glass which is capable of being imparted with an impact resisting property by a chemical strengthening treatment while it maintains a high refractivity and low specific gravity. This glass, however, is not sufficient in its .nu.d value and hence occurrence of a rainbow is not eliminated if this glass is used for an ophthalmic lens of a high diopter value. Besides, the ranges of the optical constants are so narrow that freedom in selecting optical constants is limited and it is difficult to obtain the long desired optical glass of a light weight and a high refractivity over wide ranges of optical constants.
In the U.S. Pat. No. 4,057,435, a glass of a B.sub.2 O.sub.3 -SiO.sub.2 -CaO-La.sub.2 O.sub.3 -TiO.sub.2 system has been proposed. It is, however, difficult to obtain a high refractivity of 1.75 or over in this glass. Besides, this glass is insufficient in chemical durability.
It is therefore an object of the present invention to comprehensively eliminate the above described disadventages of the prior art ophthalmic glasses and provide a novel ophthalmic and optical glass which has relatively wide ranges of optical constants, namely a refractive index (.eta.d) within the range of 1.75 to 1.84, Abbe number (.nu.d) within the range of 33 to 40 and specific gravity of 4.0 or less, and which is excellent in resistivity to devitrification and tinting and in chemical durability and is capable, if necessary, of being imparted with an impact resisting property by a chemical strengthening treatment.